gtk: force the dark variant, autostart particles

GTK4 ignores gtk-application-prefer-dark-theme, and did not follow the
portal's prefer-dark either, so Adwaita stayed light while the custom css
darkened only some surfaces - that is why pavucontrol had dark-grey labels
on dark backgrounds and a white content pane. GTK_THEME=Adwaita:dark fixes
GTK3 and GTK4 together.

Also recolour the stale ~/.themes Terminal-Dark leftovers, point gtk-4.0 at
a theme and icon set that exist, and start the particle layer from bspwmrc.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Cn3dHfbgNYdbZvzb1b3GQt

.config/rice/install.sh

@@ -208,6 +208,10 @@ post_setup() {
     [ "$DRY" = 1 ] && { note "would fix permissions, rebuild fonts, set wallpaper"; return 0; }
     command -v fc-cache >/dev/null 2>&1 && fc-cache -f >/dev/null 2>&1 && ok "font cache rebuilt"
 
+    if command -v gsettings >/dev/null 2>&1; then
+        gsettings set org.gnome.desktop.interface color-scheme 'prefer-dark' 2>/dev/null \
+            && ok "gtk colour-scheme set to prefer-dark"
+    fi
     if command -v papirus-folders >/dev/null 2>&1; then
         papirus-folders -C blue --theme Papirus-Dark >/dev/null 2>&1 && ok "papirus folders set to blue"
     fi